Towards a molecular biology of human olfaction in health and disease

Project Summary/Abstract The olfactory epithelium is the peripheral organ for smell, housing millions of primary olfactory sensory neurons. Insults such as inflammation, infection, toxins or trauma can damage the olfactory epithelium and perturb olfactory function, causing lasting anosmia, hyposmia or parosmia. We lack both a basic understanding

of what goes wrong when people lose their sense of smell, and effective therapies for sensorineural olfactory disorders, as highlighted by our current inability to treat post-COVID olfactory dysfunction impacting millions of people. Thus, there is an urgent unmet need to define mechanisms driving olfactory neuronal homeostasis and

dysfunction in humans. In rodents, each olfactory neuron harbors a unique transcriptome based upon the singular olfactory receptor it expresses, organized into coherent gene expression programs. Fixed gene expression programs specify each neuron’s identity, and flexible programs are dynamically adjusted based

upon odor exposure. These findings have enabled genome-wide characterization of odor responses in vivo in mice, across the entire olfactory neuron population. However, humans express far fewer olfactory receptors than mice, and neither their specific receptive odor repertoire nor their dynamic in vivo transcriptional variation

have been well-defined. Unlike in rodents, we know little about how gene expression is organized in human olfactory neurons, how populations of human olfactory neurons respond to defined odors, or how odor-evoked olfactory neuron activity is altered in the setting of disease. The experiments proposed here will identify

organized patterns of gene expression in olfactory neurons isolated from human biopsies, in both controls and in subjects with objective smell loss, via two specific aims: Specific Aim 1 will establish the axes of transcriptional variation in human OSNs; Specific Aim 2 will assess responses to odor at the single cell level in

the human olfactory epithelium. Olfactory mucosal biopsies from normosmic or hyposmic subjects will be analyzed using single cell RNA-sequencing. Presenting a specific odorant to subjects prior to biopsy and sequencing, an approach termed Act-seq will be employed to query the responses of the entire olfactory

neuron array to a given odor in vivo. Act-seq will be compared from normosmic or post-Covid hyposmic olfactory samples. Completion of the proposed work will (1) directly define human odor-induced alterations in olfactory cells in normal or diseased conditions, (2) provide the first in vivo human olfactory receptor de-

orphanization, and (3) produce novel datasets that will be broadly useful to the neurobiology and chemosensory research communities. These results, defining mechanisms driving olfactory neuronal homeostasis and dysfunction in humans, will form a basis for future clinical research trials aimed at promoting

recovery of olfaction.